The increasing concern for safety has resulted in a move towards providing safer materials for public and household use. A particular area of need is that of providing flame retardant products which are now required to meet certain flame retardant criteria by both local and federal governments as well as by manufacturers of such products. One particular set of conditions used as a standard for flame retardancy is set forth in Underwriters' Laboratories, Inc. Bulletin 94 which sets forth certain conditions by which materials are rated for self-extinguishing or flame retardant characteristics.
It is known that various halogen-containing and sulfur-containing flame retardant additives can be employed to render plastic products self-extinguishing or flame retardant. Such flame retardant additives are typically employed in amounts sufficient to be effective for their intended purpose; generally on the order of 5-20 weight percent based upon the weight of the plastic compositions. In many instances the use of these flame retardant additives in such amounts can have a degrading effect upon the plastic compositions to be rendered flame retardant, often resulting in the deterioration of valuable physical properties of the base polymers. This is particularly so when known flame retardant additives are employed with polycarbonate resins since many of these additives have a deleterious effect upon the carbonate polymer, typically resulting in discoloration, loss of impact strength, and loss of clarity and/or transparency.
It is known that polycarbonates based on halogen-containing diphenols and thiodiphenols, and blends of halogen-free polycarbonates based on diphenols and thiodiphenols with halogen containing diphenols are flame retardant, as disclosed in U.S. Pat. Nos. 4,043,980 and 4,174,359. However, the side effects of the halogen-containing polycarbonates such as the high processing temperatures necessary to obtain molded products, and the adverse effects on impact strength, particularly thick section impact strength, seriously limit the applications of such halogen containing polycarbonate compositions.
It is also known to provide flame retardant polycarbonate compositions by admixing with the polycarbonate resin certain organic alkali and alkaline earth metal salts of sulfonic acids. Such flame retardant compositions are disclosed in U.S. Pat. Nos. 3,909,490; 3,917,559; 3,919,167; 3,926,908; 3,933,734; 3,940,366; 3,951,910; 3,953,396; 3,953,399 and 3,978,024. While these compositions have proved to be quite effective and useful in most applications they suffer from the disadvantage that in some instances, particularly if some of these salts are used in relatively large concentrations, e.g., approaching the 5-10% levels, which range is within the 0.01-10% range disclosed by the aforementioned patents, they tend to adversely impact upon the optical transparency of the polycarbonate compositions.
It is also known, as disclosed in copending application Ser. No. 347,484, filed on Feb. 10, 1982 that polycarbonate compositions can be rendered flame retardant by either admixing with the carbonate polymer a polymer based on a thiodiphenol, or incorporating into the polycarbonate backbone a thiodiphenol residue. While these compositions are also quite effective and useful in most applications, they suffer from the disadvantage that relatively quite large amounts of thiodiphenol, typically from about 23-98 mole percent, must be employed to render said compositions flame retardant. Since thiodiphenol is relatively expensive, as compared with dihydric phenols such as bisphenol-A, its use, particularly at the upper concentration ranges, places these flame retardant polycarbonate compositions at an economic disadvantage. Secondly, in some applications, particularly those where the polycarbonate resin is required to exhibit properties of sulfur-free bisphenols such as bisphenol-A, such large concentrations of thiiodiphenol are undesirable.
It would thus be very advantageous if a flame retardant polycarbonate composition could be provided which exhibited substantially most of the advantageous properties of sulfur-free and halogen-free polycarbonates, such as for example, good impact strength, particularly good thick section impact strength, optical transparency, and good processability, while at the same time being economically competitive. It is, therefore, an object of the instant invention to provide such a flame retardant polycarbonate composition.
It has been discovered that employing minor amounts of the organic alkali or alkaline earth metal salts of sulfonic acids with a polycarbonate resin derived from a dihydric phenol and minor amounts of a thiodiphenol results in a flame retardant polycarbonate composition which exhibits, to a substantial degree, substantially most of the advantageous properties of unmodified polycarbonate resins, such as for example, good impact strength, particularly good thick section impact strength, ease of processability, and optical transparency, while simultaneously being economically competitive with unmodified dihydric phenol based polycarbonate compositions.